The present invention relates to discharge machining methods in which a workpiece is machined by applying electric current to the workpiece and the machining electrode through a machining solution in the interelectrode space between the workpiece and the electrode and to apparatuses for practicing these methods. More particularly, the invention relates to improvements in such discharge machining methods and apparatuses.
In a conventional discharge machining method, a workpiece is machined by electric discharge which is carried out between the workpiece and the machining electrode while the electrode is moved relative to the workpiece in a primary or main machining direction, hereinafter referred to as "a Z-axis direction or a Z-axis", when applicable. Then, the electrode is moved in a plane substantially perpendicular to the Z-axis, hereinafter referred to as "an X-Y plane", when applicable. The former and latter relative movements of the workpiece and the electrode are referred respectively to as "a primary machining feed" and "a secondary machining feed" hereinafter when applicable.
A discharge machining method utilizing a primary machining feed along the Z-axis and a secondary machining feed in the X-Y plane is well known as disclosed in Japanese Published Patent Application No. 3594/1966, for instance. This conventional method is advantageous in that a plurality of machining steps, namely, a coarse machining step, a middle machining step, a middle finish machining step, a finish machining step and a fine finish machining step can be carried out continuously. In general, in the coarse machining step, only the primary machining feed along the Z-axis is carried out using a large electric current as a result of which the machining gap is relatively large. As the discharge machining operation advances towards the fine finish machining step, the discharge current is gradually decreased while the machining gap is also decreased. The above-mentioned secondary machining feed in the X-Y plane makes it possible to smooth the machined surfaces with a single electrode while making up for the decrease of the machining gap.
With a conventional discharge machining apparatus utilizing a secondary machining feed, powder or chips which are created by the discharge machining and are retained in the machining gap and portions of the insulating machining solution which have been thermally decomposed by high temperature arcs during the discharge machining can be removed by the pumping action of the machining solution which is carried out in association with the secondary machining feed with the result that satisfactory surface roughness is provided for the workpiece.
An example of the conventional discharge machining method will be described with reference to FIG. 1.
FIG. 1 illustrates how an ordinary machining operation is carried out according to the conventional discharge machining method in which a workpiece 2 is machined with an electrode 1 which is scalene-triangular in section. A secondary machining feed, which is a circular motion in this case, in the X-Y plane is imparted to the electrode 1, the radius of the circular motion being indicated by R. This method provides the same effect as an electrode the radius of which is as large as the radius R which can be selected as required. However, as is apparent from FIG. 1, ech corner of the configuration which is formed on the workpiece is rounded with a radius R. That is, the configuration formed in the workpiece is considerably different from the configuration of the electrode. Thus, the method is disadvantageous in that a workpiece cannot be machined with a high accuracy.
In order to eliminate the above-described difficulty, a variety of secondary machining feed methods have been proposed in the art. One of the methods is as illustrated in FIG. 2 in which an electrode 1 is moved relative to a workpiece 2 radially and in equal length movements towards the vertices of the configuration which is to be formed in the workpiece 2. In FIG. 2, the relative displacements towards the vertices are indicated by vectors a, b and c, respectively, the magnitudes of which are all equal to R. As is clear from the configuration formed in the workpiece shown in FIG. 2, the interior angles are irregular even if the electrode is radially moved relative to the workpiece as described above. That is, the formed configuration is considerably different from that of the electrode 1. Thus, the discharge machining according to this method is still unsatisfactory in accuracy.
Another improved method is illustrated in FIG. 3. In this method, a secondary machining feed is carried out in which the three sides A, B and C of the electrode 1 are displaced relative to the workpiece 2 in a ratio k of similarity. However, this method is also disadvantageous in that the machining gaps .alpha., .beta. and .gamma. between the electrode 1 and the workpiece 2 are different from one another and therefore the configuration formed is different from the configuration of the electrode unless the electrode happens to be an equilateral triangle in section. More specifically, in the conventional method illustrated in FIG. 3, the enlargement widths .alpha., .beta. and .gamma. which are obtained from the secondary machining feeds of the electrode are different from one another. Therefore, the method suffers from a drawback in that the machined surfaces are not uniform after a plurality of machining steps from a coarse machining step to a fine machining step. That is, discharge machining with this technique does not yield satisfactory surface roughness on a workpiece.